The present invention relates to a photopolymerizable composition that comprises (1) a polymeric binder, (2) an ethylenically unsaturated, polymerizable compound, (3) a photoinitiator and (4) a particular type of photochromic spiro-indolinobenzopyran compound.
Compositions containing components (1), (2) and (3) above are used for producing printing plates, relief images and photoresists and are, for example, known from German Pat. Nos. 1,171,267, 2,027,467 and 2,039,861 and from German Offenlegungsschriften No. 15 22 515, No. 20 64 079 and No. 20 64 080 (corresponding to U.S. Pat. No. 3,804,631). They are employed, in particular, as recording materials for the photomechanical production of lithographic printing forms and for photoresist processes.
In the photopolymerizable layers of these compositions or reproduction materials, respectively, dyes or pigments are usually included to improve the visibility of the relief image following development. The colored additives should be lightfast by nature, since the stencil obtained after development, which consists of the light-hardened layer constituents, must exhibit a maximum of contrast. On the other hand, the exposed areas are clearly distinguishable from the unexposed areas immediately after the exposure of photopolymerizable recording materials. To achieve this objective, azo dyes are used, among others, which are bleached out upon exposure. In most cases the images produced using these dyes are very light, i.e., poor in contrast.
German Offenlegungsschrift No. 28 07 933 describes specific azo dyes which are reversibly bleached. When being exposed, they are bleached by down to 50% of the original extinction, resulting in a relatively high image contrast. The images must, however, be developed immediately after exposure, because, especially in the presence of ambient oxygen from the air, the original color density is gradually restored and, hence, the contrast due to exposure disappears.
In accordance with the teaching of European patent application No. 24,916, suitable dyes include triarylmethane dyes, cis- or trans-azobenzenes, thioindigo dyes, o-nitrobenzene derivatives and spiropyran compounds which in positive-working photoresists exhibit photochromic properties.
German Offenlegungsschrift No. 31 31 448 describes photopolymerizable mixtures with binders containing amino or imino groups, which may additionally contain dyes, contrast agents or photochromic compounds. Leuco dyes, such as leuco crystal violet, are mentioned as photochromic compounds. Triphenyl methane dyes and 3'-phenyl-7-dimethylamino-2,2'-spirodi[2H-1-benzopyran] are mentioned, inter alia, as suitable contrast dyes.